However, a group of scientists has now warned the scans could be dangerously misleading as they do not reflect the true nature of an unborn baby's brain.

Dr Donald Peebles, a consultant in foetal medicine at University College London, said the temptation to associate foetal movements with adult movements was "incredibly dangerous" and said they contributed nothing to the debate over whether the legal time limit for abortion should be lowered.

Dr Huseyin Mehmet, a reader in developmental neurobiology at Imperial College London, said that a foetal brain at 23 and 24 weeks was "extremely immature" and described it as being like an orange that has been sliced in half.

But Julia Millington, of the Pro-Life Alliance, said it was irrelevant whether someone looking at the images fully understood the science behind it or not.

She said people responded to the humanity of the images rather than the science behind them.

"It is accepted generally that although a foetus appears to be smiling, it is not doing so in the way we smile," she said.

"When the ultrasound images were first made public we were told it could just be a reflex response. But that doesn't change the reality that we can see how well developed an unborn baby is."

Ms Millington said she did not believe that the comments made by the scientists would significantly alter the debate over when abortions should take place.

She said: "I do not think they will diminish the impact of the images because the thing that people respond to is what they see as the humanity of the unborn child.

"Most people will recognise that a very early foetus is incapable of surviving outside the womb without being told by a scientist."